With a recent practical implementation of a blue light-emitting diode, studies are being aggressively made to develop a white light source by utilizing this diode as the light emission source. White light is in very large demand as a light source for illumination and, in addition, is greatly advantageous in that a light-emitting diode has a low power consumption and ensures a long life, compared with existing white light sources.
According to this method, the blue light emitted from the blue light-emitting diode is converted into white light by using a material having a light conversion function, in which out of three primary light colors, the blue light is included in the light emitted from the blue light-emitting diode, but green light and red light must be emitted. For this purpose, a phosphor capable of absorbing light at a certain wavelength and emitting light at a wavelength different from the absorbed light is used.
With respect to the method for converting blue light of the blue light-emitting diode into white light, as described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2000-208815, a coating layer containing a phosphor capable of absorbing a part of the blue light and emitting yellow light, and a molded layer for mixing the blue light of the light source and the yellow light from the coating layer are provided at the front of a light-emitting device. Referring to FIG. 1, a coating layer 2 is present at the front of a light-emitting device 1, and a molded layer 3 is further provided thereon. In the Figure, 4 is an electrically conducting wire, and each of 5 and 6 is a lead. In this case, color mixing takes place not only in the molded layer 3 but also in the coating layer 2.
As for the coating layer employed in conventional techniques, a mixture of an epoxy resin and a YAG (Yttrium-aluminum-garnet) powder doped with a cerium compound is coated on a light-emitting device (see, Kokai No. 2000-208815). However, according to this method, uniform white light can be hardly obtained with good reproducibility because of difficulty in control for, for example, ensuring uniform mixing of the phosphor powder and the resin, or optimizing the thickness of coated film. Also, use of a phosphor powder having low transparency to light is an obstacle to the production of a high-brightness light-emitting diode. Furthermore, heat storage arises as a problem when high-intensity light is to be obtained, and the heat resistance and the ultraviolet light resistance of resins for the coating layer and molded layer become important problems.
In order to overcome these problems, a material of emitting yellow light by absorbing blue light emitted from the light-emitting diode, and at the same time, exhibiting excellent light mixing property and high heat resistance is necessary.
Thus, it could be advantageous to provide a ceramic composite material not only having a light conversion function, that is, a function of absorbing light at a certain wavelength and emitting light at a wavelength different from the absorbed light, but also ensuring high brightness and a good light mixing property as well as excellent resistance against heat and ultraviolet light.